1. Field of the Invention
The present invention involves a recognition that there are three substantially different flow conditions or zones encountered by a finned tube and that these three different zones translate into three different heat transfer conditions; that is, assuming that a finned tube is subjected to fluid flowing from a given location and in a constant direction towards and past the finned tube, the upstream portion of the fluid flow will contact the leading edge of the fin so as to flow in an inward radial direction and at a relatively higher pressure thereby constituting a first zone; a second zone includes the areas on opposite sides of the tube where the fluid flows around the tube and across the fins substantially tangent to the side of the tube and at a pressure intermediate that of the first and third zones; a third zone is located on the trailing edge of the fin (opposite from the first zone) where the fluid moves also in a generally (outward) radial direction but with swirling vortices and relatively lower pressure.
Based upon the recognition that these three zones with their three different heat transfer conditions do indeed exist, the present invention involves enhancement (or no enhancement) of the fin in these three different zones. The fin is preferably configured differently in each zone to enhance heat transfer at the flow condition encountered in each zone. As will hereinafter appear, the fin can be serrated in a given zone, or enhanced in a given zone, or enhanced and serrated in a given zone. If it is desired to utilize an enhanced serrated fin, several different types of enhanced serrated fins are described in the above-mentioned co-pending application. One possible fin configuration to be used in one of the three zones is the elimination of the fin itself (a "no fin" configuration) if the flow rate and/or heat transfer rate is low enough and material or weight savings can be achieved.
2. The Prior Art
Heat exchange tubes are employed in a process heater or boiler. The function of the tubes is to transfer heat from spent fuel gases such as hot flue gases flowing across the outside of the tubes to a liquid, generally water or a hydrocarbon, circulating inside the finned tubes. The heated liquid is used to operate a turbine or used for other process purposes.
The transfer of thermal energy, i.e. heat, through the tube should be as efficient as possible so the amount of fuel used can be reduced. For these reasons, finned tubes are used because the fins on the tubes increase the exterior surface area of the tubes and thus increase their heat transfer capability. In reality, the recurring cost of fuel is always minimized, so the economic benefit is to reduce the cost of the equipment itself.
The exterior surface areas of prior art finned tubes have been increased by at least two means: i.e., spacing the fins closer together and providing higher fins. However, with respect to the "three zones" of the present invention, all prior art approaches towards improving the efficiency of finned tubes have involved the same style of surface in all three zones.